1. Field of the Invention
The present invention relates generally to a method of manufacturing an epitaxial wafer for use in fabricating a semiconductor device, and more particularly to a method of preventing formation of a nodule or a small needle-like projection resulting from abnormal growth of silicon, which poses a serious problem in depositing a semiconductor film on a surface of a wafer by an epitaxial growth process.
2. Description of the Related Art
FIG. 6 schematically shows a member for holding a wafer 21 on a susceptor in a conventional epitaxial growth system. When a semiconductor film is deposited on a surface of the wafer 21 by an epitaxial growth process, a back side protective film 22 consisting of an oxide or a nitride is normally formed to a thickness of about 300 nm on the back surface of the wafer 21, as shown in FIG. 6. Thereby, an impurity due to a dopant contained in the wafer is prevented from diffusing into a reaction chamber through the back surface of the wafer in the course of the epitaxial growth process. The back side protective film 22 is formed over the entire back side surface of the wafer 21. In general, in order to obtain the back side protective film, a protective film of an oxide or a nitride is, at first, formed on the top and back side surfaces of the wafer in a diffusion furnace, etc. Then, when the protective film on the top surface of the wafer surface is polished away for epitaxial growth, in order to prevent chipping at a beveled surface, the protective film 22 is normally removed from the beveled surface 26 (i.e. an arcuately beveled peripheral portion of the wafer).
When a thick epitaxial deposition film needs to be formed, SiHCl.sub.3 with a high deposition rate is used as material gas in consideration of productivity. This kind of material gas etches the wafer with HCl or Cl.sub.2 produced during the epitaxial growth. Consequently, an impurity due to a dopant contained in the wafer diffuses into the reaction chamber, and the dopant is captured in the epitaxial deposition film and the epitaxial deposition film is contaminated (so-called "auto-dope"). Thus, the function of the back side protective film 22 is very important when this material gas is used.
However, when a semiconductor film is deposited on the surface of the wafer 21 coated with the back side protective film 22 by the epitaxial growth process, a nodule 24 or a needle-like projection of silicon forms on that portion of the protective film 22, which is in direct contact with a susceptor 25, as shown in FIGS. 6 and 7. This phenomenon is considered to occur for the following reason. As is shown in FIG. 8, since the peripheral portion of the wafer 21, which is supported by the susceptor 25, is put in contact with the material gas, the material gas enters a discontinuous portion, e.g. a pore, in particles 23 of the back side protective film 22, silicon grows abnormally and becomes a needle-like projection. If the nodule 24 of silicon forms, a problem such as a focusing error or a damage to a mask due to contact with the nodule 24 will occur. The formation of the nodule 24 becomes conspicuous as the thickness of the deposited silicon film increases. For example, if the thickness of the deposited silicon film is 50 .mu.m to 100 .mu.m, the length of the nodule 24 is 50 .mu.m to 60 .mu.m.
In the prior art, the nodule 24 is mechanically polished away. As a result, the manufacturing cost increases. In addition, if the polishing operation is not exactly performed, the wafer would be damaged.